百毒不侵也曾无可救药
[băi dú bù qīn yĕ céng wú kĕ jiù yào]
Describes someone who was once extremely troubled('once incurable') yet now is untroubled by anything ('nothing can harm'). The contrast suggests growth through trials which resonate well in stories of self-improvement popular across Eastern cultures, emphasizing strength developed out hardship.